Drug metabolism Flashcards
What is the function of drug metabolism?
It is reponsible for converting lipophilic drugs to more hydrophilic compounds to facilitate their excretion
What is Drug metabolism?
A biochemical modification of pharmaceutical substances by living organisms, usually through enzymatic activity
What happens to lipid soluble substances in the kidney?
They are reabsorbed
Where are the Metabolizing enzymes present?
Liver, intestine and
blood
In a low portion on lungs
What is the function of the reactions in metabolism?
Increasing water solubility
What are the functions of drug metabolism?
- The major elimination pathway of drugs from the body
- Evolved to deal with environmental toxins
- Limits the life of a substance in the body
- Promotes excretion and reduces binding affinity for biological targets
What could happen if active metabolism are produced?
Side effects may occur or may be useful for the use of prodrugs
How many phases drug metabolism have?
Phase I, II and perphaps 3
What are the type of reactions that may happen in Phase I metabolism?
Introduces or exposes polar functional groups
Provides sites for Phase II metabolism
Addition or unmasking of: -OH, -NH2, -SH, -COOH, etc..
Where does Phase I metabolism reactions happen?
Occurs in most tissues
Primary “first pass” site of metabolism occurs during hepatic circulation
Also in gastrointestinal epithelial, renal, skin and lung tissues
Where does Phase I metabolism reactions happen (subcellular distribution)?
Most phase I enzymes are located in the endoplasmic reticulum
Enriched in microsomal preparations
Describe the phase I of metabolism. Include
- Reactions
- Hydrophilicity
- Mechanism
- Consequence
Phase I include hydrolysis, oxidation and reduction reactions. The increase in hydrophilicity is small, mechanism focus on creates functional group. Consequence, Facilitates excretion Primes
for phase II
Describe the phase II of metabolism. Include
- Reactions
- Hydrophilicity
- Mechanism
- Consequence
Phase II include conjugate reactions. The increase in hydrophilicity is large, mechanism focus on polar group added to
functional group. Consequence, Facilitates excretion.
Describe Phase II metabolism reactions
- Reactions are generally synthetic
- Almost always result in loss of biological activity
- Involves conjugation of functional groups with hydrophilic endogenous substrates
What types of groups are added during phase II metabolism and why are they added?
- Small, polar groups: glutathione, glucuronic acid, sulfate, methyl, amines/amino acids, etc..
- Increases hydrophilicity
- Conjugates are water soluble and readily excreted from the body
What types of enzymes are involved in phase II metabolism
Specify reaction and enzyme involved
- Glucuronidation - UDP-glucuronosyltransferase (UGT)
- Sulfation - sulfotransferases
- Glutathione (GSH) conjugation - glutathione S-transferases
- Acetylation - N-acetyltransferases
- Methylation - methyltransferases
What are the principal phase I enzymes?
– Cytochrome P450 (CYP) (in the liver)
– Flavin monooxygenase
– Monoamine oxidase
– Esterases
– Amidases
– Hydrolases
– Reductases, dehydrogenases, oxidases
What are the most common enzyme where the drug is metabolized in phase I and Phase II?
Phase I: CYP specific CYP3A4/5/7
Phase II: UGT
The are specific polymorphisms that may be clinically relevant. Mention the enzyme and the gene that may be affected by this
CYP: cytochrome P450
NQ01: NADPH:quinone oxidoreductase (DT diaphorase)
DPD: dihydropyrimidine dehydrogenase
ADH: alcohol dehydrogenase
ALDH: aldehyde dehydrogenase
Give some example of the drug-metabolizing enzymes that exhibit clinically
relevant genetic polymorphisms
– GST: Glutathione-S-Transferases
– ST: Sulfotransferase
– HMT: histamine methyltransferase
– COMT: catechol O-methyltransferase
– TPMT: thiopurine methyltransferase
– UGT: UDP-Glucuronosyl-S-Transferases
What assay may be used to identify metabolites produced during metabolism?
Microsome or hepatocyte assay coupled with LC-MS to identify major metabolites
What questions should be asked when you identify certain metabolites for phase II metabolism
– What metabolites are formed?
– Are they active?
– Are they reactive/toxic?
* Should these be considered in a Target Product Profile?
What are “soft drugs”?
Projected to be safer drugs with an increased therapeutic index
– Limit the duration of action by integrating metabolic liabilities into the drug
What happens after absorption to soft drugs?
They are rapidly metabolized to components that are quickly eliminated from the body
Undergo predictable, “controllable” metabolism to nontoxic and
inactive metabolites
– Generally avoid oxidative metabolism
– Use hydrolytic enzymes to achieve predictable and controllable drug
metabolism
* Commonly plasma and tissue esterases
what is a hard drug and how it is excreted?
Hard drugs are non- metabolisable drugs- usually to liphophilic.
1.Simplified pharmacokinetics
- Excreted primarily through bile or kidney unchanged
2. Removes toxicity due to reactive or active metabolites
Give 2 example of hard drugs
-Zoledronic acid (osteoporosis)
-Lisinopril (hypertension)
What is Atracurium?
- A nicotinic acetylcholine receptor antagonist
– A non-depolarizing muscle relaxant used during surgery or mechanical ventilation
– Requires fast recovery time - Undergoes spontaneous Hoffman elimination and ester hydrolysis at
physiological pH
Describe Remifentanil
- Novel, short-acting -opioid receptor agonist used during surgery
– Rapid onset and recovery time - Undergoes rapid hydrolysis by non-specific tissue and plasma
esterases to remifentanilic acid
– 1/4600th the potency of remifentanil - Half-life remains at 4 min, even after a 4 h infusion
What is the purpose of a prodrug?
Most common reason to use a pro-drug approach is to increase
oral absorption and hence F
What are active metabolites?
Result when a drug is metabolised into a modified form that continues to produce effects in the body.
When are the active metabolites formed? Give an example
Pharmacologically active metabolites are generally formed by phase I
oxidative reactions
Example
- Acetaminophen (paracetamol) is an O-deethylated metabolite of phenacetin
– Superior analgesic activity with fewer side-effects
However, in phase II conjugation reactions can also produce biologically active metabolites- better
safety profiles
Example
-Morphine 6-glucuronide is more potent as a -opioid receptor agonist than
morphine
Where does conversion of a prodrug happen in the body?
Conversion of pro-drug to active metabolite can occur in small
intestine or liver (plasma, lung)
What are the desired characteristics of a prodrug?
– Limited target engagement
– Rapid hydrolysis to the active compound
– High target engagement of the active compound (primary metabolite)
– No pharmacological effect of secondary metabolites
What are the reactions that are include to form pro-drugs?
-Esterification
– Hydrolysis
– Phosphorylation
– Oxidation
– Reduction
What model could be used to describe metabolite kinetics
Two compartment model for metabolite kinetics
– Compartment models can also be used to evaluate metabolite
kinetics
Give an example of pro-drugs. How is it form?
– Pivampicillin, talampicillin, and bacampicillin are pro-drugs of ampicillin
(BAV < 50 %)
– All result from the esterification of the polar carboxylate group to form
lipophilic, enzymatically labile esters (F = 98-99 %)
Why is it important to understand the kinetics of drug metabolites?
– Can be very important to understand
* Potency of metabolites – prodrugs for example
– Need to consider metabolites for secondary pharmacology
– Understanding metabolite kinetics is also of great importance in
toxicology
What is fm and k.fm in the image? what type of model it represent? and which is the plasma compartment?
fm: A fraction of the drug
k.fm: rate constant
Parent drug is the center compartment- plasma compartment.
Describe two compartment model for metabolite kinetics
What compartments, equations and assumptions
= (parent → metab) – elimination
= 𝑘. 𝑓𝑚. 𝐴𝑏 − 𝐴𝑚. 𝑘𝑚
- NB Assume that metabolite and parent are both
excreted by the same route
->elimination of parent
= total elimination (i.e. elimination of converted +
elimination of non-converted)
What is the cofactor of cytochrome P450?
Haem
What is the absorbance of CYP450?
Mention why the absorbance is at that specific wavelenght
They have an unusual absorbance maximum at 450 nm upon C=O binding to the reduced form (Fe2+) of the haem
– This led to the initial P450 designation
* P is for pigment
* Other haem proteins have λmax at 420 nm
* Absorbance at 450 nm is due to an unusual
fifth ligand to the haem: a cysteine-thiolate
Mention 3 examples of CYP450 families, its function and some of their names
- CYP1
Drug and steroid (especially estrogen) metabolism,
benzo[a]pyrene toxification
3 subfamilies, 3 genes, 1 pseudogene
CYP1A1, CYP1A2, CYP1B1 - CYP2
Drug and steroid metabolism 13 subfamilies, 16 genes, 16 pseudogenes
CYP2A6, CYP2A7, CYP2A13, CYP2B6, CYP2C8, CYP2C9, CYP2C18, CYP2C19, CYP2D6, CYP2E1, CYP2F1, CYP2J2, CYP2R1, CYP2S1, CYP2U1, CYP2W1 - CYP3
Drug and steroid (including testosterone) metabolism 1 subfamily, 4 genes, 2 pseudogenes
CYP3A4, CYP3A5, CYP3A7, CYP3A43
Explain each color letter of the image.
Cytochrome P450
has more than 50
enzymes, six of them metabolize 90 % of drugs. Which are the most significant?
Two most significant enzymes are CYP3A4
and CYP2D6
Who do CYP450 affect drug-drug interactions?
Many drugs may increase (or induce) or decrease (or inhibit)
the activity of various CYP isozymes
* For example, if drug A inhibits the CYP-mediated metabolism
of drug B, drug B may accumulate within the body to toxic
levels
* These drug interactions may necessitate dosage adjustments
or choosing alternative drugs that do not interact with the CYP
system
What is the main reaction that is catalyzed by CYP450?
- Primarily mono‐oxygenation
– One atom of oxygen is incorporated into a substrate
– The other is reduced to H2O with reducing equivalent derived from NADPH
RH + O=O + H+ + NADPH => ROH + H2O + NADP+
What are the function of Cytochrome P450?
Cytochrome P450 are involved in metabolism of diverse endogenous compounds. Also essential for the metabolism of many
medicinal drugs.
What are the CYP450 reactions involved in Phase I metabolism?
- Hydroxylation of an aliphatic or aromatic carbon
- Epoxidation of a double bond
- Heteroatom (S-, N-) oxidations
- Heteroatom (O-, S-, N-) dealkylation
- Oxidative group transfer
- Cleavage of esters
- Dehydrogenation
What are the 3 important CYP450 activities?
- Significant divergence across species
2.Concentrated in the liver: extrahepatic enzyme activities also contribute to patho/physiological
process
3.Located in microsomes: Possibility to perform detailed metabolic studies in vitro using human microsomes
Possibility to perform detailed metabolic studies in vitro using human microsomes. Give 2 examples.
1.CYP2B6 induced by phenytoin
2. CYP1A2 induced by broccoli
What happens in carbon hydroxilation?
- Metabolic liability related to ease of hydrogen atom abstraction
and energy of resulting radical (= lower bond strength) - Adjacent benzyl group stabilises radical through delocalisation
- Can occur due to orbital overlap
Give the name and explain the following reaction.
Carbon hydroxylation, also known as C-oxidation.
-Generally accepted mechanism involves hydrogen atom abstraction
by (FeO)3+ followed by oxygen insertion (radical recombination)
true or false, alkyl sites are more labile than enzylic/allylic sites
False
Which enzyme is associate with the following process?
The enzyme associate is CYP8A1: PGI2 synthase
-Extrahepatic CYP450- Associated with endothelium microsomes
Give an example of FMO activity and Heteroatom (S-, N-) oxidation
Name the following reaction
Heteroatom (S-, N-) oxidation
Give 3 opioid examples, include the main metabolic pathway and active metabolites.
